Women who've ever smoked have an almost 20 percent increased risk for colon cancer, compared with women who never smoked, according to the new study, published April 30 in Cancer Epidemiology, Biomarkers & Prevention.

"Women who smoke even 10 or fewer cigarettes a day increase their risks for colon cancer," said lead researcher Dr. Inger Gram, a professor in the department of community medicine at the University of Tromso in Norway.

"Because colon cancer is such a common disease, even this moderate smoking accounts for many new cases," she said. "A lot of colon cancer can be prevented if people don't smoke -- especially women."

The study involved data on more than 600,000 men and women, aged 19 to 67, surveyed by the Norwegian Institute of Public Health. Participants answered questions about their smoking habits, physical activity and other lifestyle factors.

Over 14 years of follow-up nearly 4,000 people developed colon cancer, and the odds were greatest for smokers, women in particular. The risk for colon cancer increased 19 percent among women who smoked and 8 percent for men who smoked, according to Gram's team.

The more years a woman smoked, the earlier she started smoking, and the more packs of cigarettes smoked a year, the greater her risk of developing colon cancer. Women who smoked for 40 years or more increased their risk for colon cancer almost 50 percent, the researchers said.

Their risk was especially high for developing proximal, or right-sided, colon cancer, with a type of tumor specifically related to smoking, Gram noted.

Gram said she was surprised the link between smoking and colon cancer was so much greater for women, and said the reasons aren't clear.

Although this study shows an association between smoking and colon cancer, it does not establish a cause-and-effect relationship. However, the link between smoking and colon cancer is more than a coincidence, Gram pointed out.

"Colon cancer is a smoking-related cancer," she said. "That has recently been established by the International Agency for Research on Cancer of the World Health Organization." Based on a review of prior research, the WHO says long-term smoking appears to double the risk of colon cancer. It also increases risk for bladder and pancreatic cancer, according to the agency.

One expert, Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, doesn't believe the heightened risk for colon cancer among women is solely related to smoking. Alcohol use, diet and lack of exercise may also play a role, Bernik said.

"Usually, smoking goes along with other bad health habits," Bernik said. "However, this adds to the growing data that cigarette smoking contributes to the increased risk of colon cancer."

Another expert offered some advice. "If you smoke, you should quit," said Dan Jacobsen, from the Center for Tobacco Control at North Shore-LIJ Health System in Great Neck, N.Y. "There are a lot of good methods, programs and resources out there if you want to try to quit smoking," he added.

"Smoking is just toxic to our bodies," said Jacobsen. "It's the number one preventable cause of death and disease."

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